With the increasing reliability of the details in elastic global models, it may be possible, in the near future, to accurately predict elastic effects on the amplitudes of globally traveling long period surface and body waves, thereby gaining access to more accurate estimates of the lateral variations of Q in the earth.
In this preliminary study, we experiment with a waveform formalism, based on mode summation by path average method, applied to surface waves. Starting with the SH model, SAW12D (Li and Romanowicz, 1996), we invert iteratively for Q structure. The Q model is parameterized radially in terms of layers, and we progressively increase the maximum degree of the lateral expansion in spherical harmonics.
To model the Q heterogeneity with waveform inversion progressively, only mantle Love waves of the first and second orbits (G1 and G2) are considered in this study. The SH accelerograms are low-pass filtered with a cutoff frequency of 1/80 Hz and a corner frequency 1/100 Hz.
11353 wave packets with good global coverage (Figure 30.1)are used in this study.
Images of our degree 5 Q model in upper mantle layers are shown in Figure 30.2. The main features in this model are summarized in three parts:(1) The pattern in the uppermost mantle is well correlated with tectonic features. (2) the high Q in the Eurasian Plate and northern America and low Q in east pacific region are quite stable over different depths, while the low attenuation in northern mid-Atlantic region fades out at depth below 220km. (3) the amplitude of heterogeneity decreases with depth.
Li, X.D. and B. Romanowicz, Global mantle shear-velocity model developed using nonlinear asymptotic coupling theory, Geophys. J. R. Astr. Soc., 101, 22,245-22,272, 1996.
Richard, M. A., B. H. Hager, and N. H. Sleep, Dynamically supported geoid highs over hotspots: observation and theory,J. Geophys. Res., 93 7690-7708.